Process for producing terephthalic acid

ABSTRACT

Methods of producing terephthalic acid are described. The methods involve using a p-xylene stream enriched with p-toluic acid. The p-xylene stream enriched with p-toluic acid, a solvent comprising an ionic liquid and optionally a carboxylic acid, a bromine source, a catalyst, and an oxidizing agent are contacted to produce a product comprising terephthalic acid.

FIELD OF THE INVENTION

This invention relates to terephthalic acid compositions and processesfor producing terephthalic acid from a feedstock comprising para-xylene.More particularly, the invention relates to the oxidation of a p-xylenestream enriched with p-toluic acid to produce terephthalic acid.

BACKGROUND OF THE INVENTION

Oxidation of alkyl aromatic compounds, e.g., toluene and xylenes, areimportant commercial processes. A variety of oxidation products may beobtained including aromatic carboxylic acids such as terephthalic acid(1,4-benzenedicarboxylic acid) and isophthalic acid(1,3-benzenedicarboxylic acid) which are used, for example, in thepolymer industry.

It is known that oxidation products, such as aromatic alcohols, aromaticaldehydes, aromatic ketones, and aromatic carboxylic acids, may solidifyor crystallize at oxidation conditions and/or as the reaction mixturecools. Thus, mixtures of oxidation products may be produced whichrequire further processing to increase the purity of the desiredproduct. For example, in the production of terephthalic acid, theoxidation product is often referred to as crude terephthalic acidbecause it contains impurities including color bodies and intermediateoxidation products, especially 4-carboxybenzaldehyde (4-CBA). To obtainpolymer grade or purified terephthalic acid, various purification stepsare known in the art including: washing the crude terephthalic acid withwater and/or a solvent, additional oxidation or crystallization steps,and reacting a solution of dissolved crude terephthalic acid withhydrogen at hydrogenation conditions usually including a catalystcomprising palladium and carbon. Often several purification steps areused.

U.S. Pat. No. 2,833,816 discloses processes for oxidizing aromaticcompounds to the corresponding aromatic carboxylic acids. A process forthe liquid phase oxidation of alkyl aromatic compounds uses molecularoxygen, a metal or metal ions, and bromine or bromide ions in thepresence of an acid. The metals may include cobalt and/or manganese.Exemplary acids are lower aliphatic mono carboxylic acids containing 1to 8 carbon atoms, especially acetic acid.

U.S. Pat. No. 6,355,835 discloses a process for the preparation ofbenzene dicarboxylic acids by liquid phase oxidation of xylene isomersusing oxygen or air by oxidizing in the presence of acetic acid as asolvent, a cobalt salt as a catalyst, and an initiator. The oxidationstep is followed by flashing the reaction mixture to remove volatilesubstances and cooling and filtering the material to get crude benzenedi-carboxylic acid as a solid product and a filtrate. Recrystallizingthe crude benzene di-carboxylic acid to obtain at least 99% purity andrecycling of the filtrate are also disclosed.

U.S. Pat. No. 7,094,925 discloses a process for preparing analkyl-aromatic compound. The process includes mixing an oxidizing agentor sulfur compound in the presence of an ionic liquid. Air, dioxygen,peroxide, superoxide, or any other form of active oxygen, nitrite,nitrate, and nitric acid or other oxides or oxyhalides of nitrogen(hydrate or anhydrous) can be used as the oxidizing agent. The processis typically carried out under Bronstead acidic conditions. Theoxidation is preferably performed in an ionic liquid containing an acidpromoter, such as methanesulfonic acid. The product is preferably acarboxylic acid or ketone or intermediate compound in the oxidation,such as an aldehyde, or alcohol.

U.S. Pat. No. 7,985,875 describes a process for preparing an aromaticpolycarboxylic acid by liquid phase oxidation of a di- ortri-substituted benzene or naphthalene compound. The process involvescontacting the aromatic compound with an oxidant in the presence of acarboxylic acid solvent, a metal catalyst, and a promoter in a reactionzone. The promoter is an ionic liquid comprising an organic cation and abromide or iodide anion. The promoter is used in a concentration rangeof about 10 to about 50,000 ppm (based on solvent) with a preferredrange of 10-1,000 ppm. No other promoters, such as bromine-containingcompounds, need to be used in the process. The process produces crudeterephthalic acid (CTA) having 1.4-2.2% 4-CBA. Purification of the CTAis required to obtain purified terephthalic acid (PTA).

US 2010/0174111 describes a process for purifying aryl carboxylic acids,such as terephthalic acid. The impure acid is dissolved or dispersed inan ionic liquid. A non-solvent (defined as a molecular solvent for whichthe ionic solvent has high solubility and for which the aryl carboxylicacid has little or no solubility) is added to the solution toprecipitate the purified acid.

U.S. Pat. No. 7,692,036, 2007/0155985, 2007/0208193, and 2010/0200804disclose a process and apparatus for carrying out the liquid-phaseoxidation of an oxidizable compound. The liquid phase oxidation iscarried out in a bubble column reactor that provides for a highlyefficient reaction at relatively low temperatures. When the oxidizedcompound is para-xylene, the product from the oxidation reaction is CTAwhich must be purified. Purification is said to be easier than forconventional high temperature processes.

SUMMARY OF THE INVENTION

The invention provides a process for the oxidation of para-xylene toterephthalic acid. It has been discovered that the invention may be usedto produce terephthalic acid compositions having different amounts ofcontaminants relative to those observed in conventional processes. Insome embodiments, the terephthalic acid compositions produced by theprocess have low levels of impurities and thus are less costly topurify. In some embodiments, the invention produces polymer grade orpurified terephthalic acid without the use of catalytic hydrogenation.

One aspect of the invention is a process for producing terephthalicacid. In one embodiment, the process includes providing a p-xylenestream enriched with p-toluic acid; and contacting the p-xylene streamenriched with p-toluic acid, a solvent comprising an ionic liquid, abromine source, a catalyst, and an oxidizing agent to produce a productcomprising terephthalic acid.

In another embodiment, the process includes providing a p-xylene streamcomprising p-toluic acid, wherein the p-xylene stream is produced by aprocess in which the p-toluic acid is maintained at a level of at leastabout 5% by weight of the p-xylene stream produced therefrom; contactinga solvent comprising an ionic liquid, a bromine source, a catalyst, thep-xylene stream of said providing step, and an oxidizing agent, toproduce a product comprising terephthalic acid.

In another embodiment, the process includes contacting p-xylene, a firstsolvent, a first bromine source, a first catalyst, and a first oxidizingagent to produce a p-xylene stream enriched with p-toluic acid;maintaining the p-toluic acid at a level of at least about 5% by weightof the p-xylene stream enriched with p-toluic acid; and contacting thep-xylene stream enriched with p-toluic acid, a second solvent comprisingan ionic liquid, a second bromine source, a second catalyst, an a secondoxidizing agent to produce a product comprising terephthalic acid.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a reaction scheme for the oxidation of p-xylene.

FIG. 2 is a graph showing the effect of starting material on levels ofimpurities.

DETAILED DESCRIPTION OF THE INVENTION

In general, the invention relates to terephthalic acid compositions andprocesses for the oxidation of para-xylene to terephthalic acid. Inbroad terms, the invention is a process for producing terephthalic acidfrom a p-xylene stream enriched with p-toluic acid.

As discussed below, it was discovered that the production of 4-CBA issignificantly lower if the starting material is p-toluic acid ratherthan p-xylene. As a result, it was determined that starting with ap-xylene feed stream enriched with p-toluic acid would be desirable.

In addition, the general reaction scheme for the production ofterephthalic acid through liquid-phase oxidation of p-xylene is shown inFIG. 1. In that series of reactions, the production of p-toluic acidfrom p-xylene is faster in acetic acid than it is in ionic liquid.However, the subsequent reactions are faster and more selective in anionic liquid/acetic acid mixture than in acetic acid alone.

The p-xylene stream enriched with p-toluic acid can be obtained in anysuitable manner, including but not limited to, oxidation, alkylation andthe like. For example, a stream having the required p-toluic acidcontent could be generated as a product stream in a petroleum orchemical processing complex. If the initial product stream does not havethe needed purity, it can be purified to the desired level using knownmethods.

The p-xylene stream enriched with p-toluic acid contains at least about5% and less than 90% by weight p-toluic acid. It typically contains atleast about 10% by weight, or at least about 15% by wt, or at leastabout 20% by weight, or at least about 25% by wt, or at least about 30%by weight, or at least about 35% by wt, or at least about 40% by weight,or at least about 45% by wt, or at least about 50% by weight, or atleast about 55% by wt, or at least about 60% by wt, or at least about65% by weight, or at least about 70% by wt, or at least about 75% by wt.It typically contains less than about 85% wt, or less than about 80% wt.

The p-toluic acid level is maintained in solution, as colloids, or as aslurry.

The basic step of the process involves contacting the p-xylene streamenriched with p-toluic acid (however obtained), a solvent comprising anionic liquid, a bromine source, a catalyst, and an oxidizing agent toproduce a product comprising terephthalic acid.

The contacting step(s) may be practiced in laboratory scale experimentsthrough full scale commercial operations. The process may be operated inbatch, continuous, or semi-continuous mode. The contacting step can takeplace in various ways. The order of addition of the components (e.g.p-xylene stream enriched with p-toluic acid, solvent, bromine source,catalyst, and oxidizing agent) is not critical. For example, thecomponents can be added individually, or two or more components may becombined or mixed before being combined or mixed with other components.

The solvent comprises at least one ionic liquid. Two or more ionicliquids can be used, if desired.

Generally, ionic liquids are non-aqueous, organic salts composed of ionswhere the positive ion is charge balanced with a negative ion. Thesematerials have low melting points, often below 100° C., undetectablevapor pressure, and good chemical and thermal stability. The cationiccharge of the salt is localized over hetero atoms, and the anions may beany inorganic, organic, or organometallic species.

Most ionic liquids are formed from cations that do not contain acidicprotons. The synthesis of ionic liquids can generally be split into twoparts: formation of the desired cation, and anion exchange to form thedesired product. Quaternization of an amine or phosphine, for example,is the initial step in the synthesis of the cation of an ionic liquid.If it is not possible to form the desired anion directly by thequaternization reaction, a further step is required.

There are estimated to be hundreds of thousands of simple ioncombinations to make ionic liquids, and an almost endless (10¹⁸) numberof potential ionic liquid mixtures. This implies that it should bepossible to design an ionic liquid with the desired properties to suit aparticular application by selecting anions, cations, and mixtureconcentrations. Ionic liquids can be adjusted or tuned to provide aspecific melting point, viscosity, density, hydrophobicity, miscibility,etc. for specific applications. The thermodynamics and reaction kineticsof processes carried out in ionic liquids are different from those inconventional media. This creates new opportunities for catalyticreactions, separations, combined reaction/separation processes, heattransfer agents, hydraulic fluids, paint additives, electrochemistryapplications, as well as many others. Ionic liquids do not emit volatileorganic compounds (VOCs), providing a basis for clean manufacturing,e.g., “green chemistry.”

The organic cation can comprise a linear, branched, or cyclicheteroalkyl unit. The term “heteroalkyl” refers to a cation comprisingone or more heteroatoms chosen from nitrogen, oxygen, sulfur, boron,arsenic, boron, antimony, aluminum, or phosphorous capable of forming acation. The heteroatom can be a part of a ring formed with one or moreother heteroatoms, for example, pyridinyl, imidazolinyl rings, that canhave substituted or unsubstituted linear or branched alkyl unitsattached thereto. In addition, the cation can be a single heteroatomwherein a sufficient number of substituted or unsubstituted linear orbranched alkyl units are attached to the heteroatom such that a cationis formed.

Non-limiting examples of heterocyclic and heteroaryl units that can bealkylated to form cationic units include imidazole, pyrazoles,thiazoles, isothiazoles, azathiozoles, oxothiazoles, oxazines,oxazolines, oxazaboroles, dithiozoles, triazoles, selenozoles,oxahospholes, pyrroles, boroles, furans, thiphenes, phospholes,pentazoles, indoles, indolines, oxazoles, isothirazoles, tetrazoles,benzofuran, dibenzofurans, benzothiophenes, dibenzothoiphenes,thiadiazoles, pyrdines, pyrimidines, pyrazines, pyridazines,piperazines, piperidines, morpholines, pyrans, annolines, phthalazines,quinazolines, and quinoxalines.

The anionic portion of the ionic liquid can comprise an inorganic,organic, or organometallic moiety. Non-limiting examples of anionsinclude inorganic anions: halogens, (e.g., F, Cl, Br, and I); borides,BX₄, wherein X represents halogen, (e.g., BF₄, BCl₄), and the like;phosphates(V), PX₆; PF₆, and the like; arsenate(V), AsX₆; AsF₆, and thelike; stibate(V) (antimony), SbX₆; SbF₆, and the like; CO₃ ²⁻; NO₂ ¹⁻,NO₃ ¹⁻, SO₄ ²⁻, PO₄ ³⁻, (CF₃)SO₃ ¹⁻ and their derivatives.

Other non-limiting examples of ionic liquid anions include substitutedazolates, that is, five membered heterocyclic aromatic rings that havenitrogen atoms in either positions 1 and 3 (imidazolates); 1, 2, and 3(1,2,3-triazolates); or 1, 2, 4 (1,2,4-triazolate). Substitutions to thering occur at positions that are not located in nitrogen positions(these are carbon positions) and include CN (cyano-), NO₂ (nitro-), andNH₂ (amino) group appended to the heterocyclic azolate core.

Further non-limiting examples of anions include substituted orunsubstituted borides: B(R)₄; substituted or unsubstituted sulfates:(RO)S(═O)₂O; substituted or unsubstituted acyl units RCO₂, for example,acetate CH₃CO₂, proprionate, CH₃CH₂CO₂, butyrate CH₃CH₂CH₂CO₂, andbenzylate, C₆H₅CO₂; substituted or unsubstituted phosphates:(RO)₂P(═O)O; substituted or unsubstituted carboxylates: (RO)C(═O)O;substituted or unsubstituted azolates wherein the azolate can besubstituted on a carbon atom by a unit chosen from cyano, nitro, andamino. R can be an organic, inorganic, or organometallic group.Non-limiting examples of R include hydrogen; substituted orunsubstituted linear branched, and cyclic alkyl; substituted orunsubstituted linear, branched, and cyclic alkoxy; substituted orunsubstituted aryl; substituted or unsubstituted aryloxy; substituted orunsubstituted heterocyclic; substituted or unsubstituted heteroaryl;acyl; silyl; boryl; phosphino; amino; thio; and seleno.

In an embodiment, ionic liquids suitable for use include, but are notlimited to, one or more of imidazolium ionic liquids, pyridinium ionicliquids, tetra alkyl ammonium ionic liquids, and phosphonium ionicliquids. More than one ionic liquid may be used. Imidazolium,pyridinium, and ammonium ionic liquids have a cation comprising at leastone nitrogen atom. Phosphonium ionic liquids have a cation comprising atleast one phosphorus atom. In an embodiment, the ionic liquid comprisesa cation selected from alkyl imidazolium, di-alkyl imidazolium, andcombinations thereof. In another embodiment, the ionic liquid comprisesan anion selected from of halides, acetate, carboxylates, andcombinations thereof. The ionic liquid may comprise at least one of1-butyl 3-methyl imidazolium acetate (BMImOAc), 1-butyl 3-methylimidazolium bromide (BMImBr), 1-hexyl 3-methyl imidazolium acetate, and1-hexyl 3-methyl imidazolium bromide.

The ionic liquid can be provided, or it can be generated in situ fromappropriate precursors, or both. If it is generated in situ, the solventcomprises precursors of one or more ionic liquids. The ionic liquidprecursors comprise a cation precursor, such as an alkyl imidazole,alkyl pyridine, alkyl amine, alkyl phosphine, and the like, and an anionprecursor, such as alkyl or aryl halides or acetates. In an embodiment,the precursors are methyl imidazole and butyl bromide.

The mode of introducing the ionic liquid precursors may vary dependingon the nature of the alkyl aromatics being oxidized and the nature andpurity of the product desired. In one mode of addition, the cationprecursors and the anion precursors (generally liquids at roomtemperature and pressure) are mixed with a carboxylic acid (for example,acetic acid) solvent and introduced into the oxidation reactor(s). Inanother mode of addition, the ionic liquid precursors may be mixed withthe alkyl aromatic feed and introduced into the oxidation reactors. Inanother mode of addition, both cation and anion ionic liquid precursorcomponents may be introduced into the bottom of the reactor withoutpre-mixing with any other oxidation reactor components such as the feed,carboxylic acid solvent, and catalyst package.

The solvent can also comprise a carboxylic acid. When carboxylic acidsare used in the solvent, the amount of carboxylic acid is decreasedcompared with conventional processes in order to avoid excessive solventvolumes. The carboxylic acid desirably has from 1 to 7 carbon atoms. Inan embodiment, the carboxylic acid comprises acetic acid. The solventmay contain more than one carboxylic acid. For example, the solvent mayfurther comprise benzoic acid. In another embodiment, the carboxylicacid of the solvent is acetic acid.

In an embodiment, the solvent has a ratio of the carboxylic acid to theionic liquid within a range of about 1:16 to 16:1 by weight, or about1:9 to 9:1 by weight, or about 3:17 to 17:3 by weight, or about 1:4 to4:1 by weight, or about 1:3 to 3:1 by weight, or about 3:7 to 7:3 byweight, or about 7:13 to 13:7 by weight, or about 2:3 to 3:2 by weight,or about 9:11 to 11:9 by weight, or about 1:1 by weight. In anembodiment, the solvent contains more than 5% by weight ionic liquid, orat least about 6% by weight ionic liquid, or at least about 10% byweight ionic liquid, or at least about 15% by weight ionic liquid, or atleast about 20% by weight ionic liquid, or at least about 25% by weightionic liquid, or at least about 30% by weight ionic liquid, or at leastabout 35% by weight ionic liquid, or at least about 40% by weight ionicliquid, or at least about 45% by weight ionic liquid. The amount ofionic liquid includes ionic liquid precursors, if present. The optionalionic solid or material capable of forming an ionic salt in solutiondiscussed below, if present, is included in the amount of ionic liquid.

Optionally, an ionic solid, such as ammonium acetate (NH₄OAc) and/orammonium bromide (NH₄Br), can be added to the mixture. Alternatively, amaterial which is capable of forming an ionic salt in solution can beadded. The material can form the ionic salt in solution by combiningwith ions present in the solution. For example, in a solution containingbromide (for example in the form of HBr) or acetate ions (for example,in the form of acetic acid), ammonia could combine with the bromide oracetate ions forming ammonium bromide or ammonium acetate. The use ofone or more ionic solids or materials capable of forming an ionic saltin solution provided an additional reduction in the level of impurities.

In an embodiment, the amount of ionic solid and material capable offorming an ionic salt in solution ranges from about 5 wt % to about 45wt %, relative to the weight of the solvent, or from about 10 wt % toabout 45 wt %, relative to the weight of the solvent. The solventincludes the carboxylic acid, the ionic liquid and/or ionic liquidprecursors, the optional ionic solid or material capable of forming anionic salt in solution, and the optional water.

Optionally, the solvent may further comprise water. The water may beadded to the mixture or generated in the mixture during the oxidationprocess. In an embodiment, the amount of water ranges from about 0.01 wt% to about 5 wt %, relative to the weight of the carboxylic acid. Theamount of water may range from about 0.1 wt % to about 2 wt %, relativeto the weight of the carboxylic acid.

In an embodiment, the ratio of solvent to p-xylene stream enriched withp-toluic acid in the mixture ranges from about 1:1 to about 10:1 byweight, or from about 1.5:1 to about 6:1 by weight, or from about 2:1 toabout 4:1 by weight. The solvent includes the carboxylic acid, the ionicliquid and/or ionic liquid precursors, the optional ionic solid ormaterial capable of forming an ionic salt in solution, and the optionalwater.

The catalyst comprises at least one of cobalt, manganese, titanium,chromium, copper, nickel, vanadium, iron, molybdenum, tin, cerium andzirconium. In an embodiment, the catalyst comprises cobalt andmanganese. The metal may be in the form of an inorganic or organic salt.For example, the metal catalyst may be in the form of a carboxylic acidsalt, such as, a metal acetate and hydrates thereof. Exemplary catalystsinclude cobalt (II) acetate tetrahydrate and manganese (II) acetate,individually or in combination. In an embodiment, the amount ofmanganese (II) acetate is less than the amount of cobalt (II) acetatetetrahydrate by weight.

The amount of catalyst used in the invention may vary widely. Forexample, the amount of cobalt may range from about 0.001 wt % to about 2wt % relative to the weight of the solvent. In an embodiment, the amountof cobalt ranges from about 0.05 wt % to about 2 wt % relative to theweight of the solvent. The amount of manganese may range from about0.001 wt % to about 2 wt % relative to the weight of the solvent. In anembodiment, the amount of manganese ranges from about 0.05 wt % to about2 wt % relative to the weight of the solvent. In another embodiment, theratio of cobalt to manganese ranges from about 3:1 to about 1:2 byweight on an elemental metal basis.

Bromine sources are generally recognized in the art as being catalystpromoters and include bromine, ionic bromine, e.g. HBr, NaBr, KBr,NH₄Br; and/or organic bromides which are known to provide bromide ionsat the oxidation conditions, such as, benzylbromide, mono anddi-bromoacetic acid, bromoacetyl bromide, tetrabromoethane, ethylenedi-bromide. In an embodiment, the bromine source comprises or consistsessentially of or consists of hydrogen bromide. The amount of hydrogenbromide may range from about 0.01 wt % to about 5 wt %, relative to theweight of the solvent. In another embodiment, the amount of hydrogenbromide ranges from about 0.05 wt % to about 2 wt %, relative to theweight of the solvent. The solvent includes the carboxylic acid, theionic liquid and/or ionic liquid precursor, the optional ionic solid ormaterial capable of forming an ionic salt in solution, and the optionalwater.

Suitable oxidizing agents for the process provide a source of oxygenatoms to oxidize the p-xylene and/or p-toluic acid, and/or anotherintermediate oxidization product at the oxidation conditions employed.Examples of oxidizing agents include peroxides, superoxides, andnitrogen compounds containing oxygen such as nitric acids. In anembodiment, the oxidizing agent is a gas comprising oxygen, e.g. air,carbon dioxide, and molecular oxygen. The gas may be a mixture of gases.The amount of oxygen used in the process is preferably in excess of thestoichiometric amount required for the desired oxidation process. In anembodiment, the amount of oxygen contacted with the mixture ranges fromabout 1.2 times the stoichiometric amount to about 100 times thestoichiometric amount. Optionally, the amount of oxygen contacted withthe mixture may range from about 2 times the stoichiometric amount toabout 30 times the stoichiometric amount.

At least a portion of the components provides a liquid phase, althoughdissolution of one or more of the mixture components may not be completeat any or some time during the process. The liquid phase may be formedby mixing the components at ambient conditions. In another embodiment,the liquid phase is formed as the temperature of the mixture increasesto the oxidation temperature. A mixture of the components may be formedprior to the oxidation step, in the same or different vessel as thatused in the oxidation step. In another embodiment, a mixture of thecomponents is formed in an oxidation reactor, e.g. adding variousstreams of the components individually and/or in combination to acontinuous or semi-continuous oxidation reactor. The combinedcomponents, and/or various streams of the components may be heatedbefore they are mixed together.

Though many conventional alkyl aromatic oxidation processes aretypically conducted in a mixed phase, and often include three phases(e.g. solid, gas, and liquid), they are frequently referred to in theart as “liquid phase” oxidation processes because the oxidationconditions are maintained to provide at least a portion of the mixturein the liquid phase. It is also known in the art that the number ofphases present may vary over time during the process. Processesaccording to the instant invention may also be conducted in a liquidphase or mixed phase in a similar manner as known in the art.

Conventional, liquid phase oxidation reactors as known in the art may beused to practice the invention. Examples include vessels, which may haveone or more mechanical agitators, and various bubble column reactorssuch as those described in U.S. Pat. No. 7,692,036. It is also known todesign, operate, and control such reactors and the oxidation reactionfor the oxidation conditions employed including, e.g., the temperature,pressure, liquid and gas volumes, and corrosive nature of the liquid andgas phases where applicable. See, e.g. U.S. Pat. No. 7,692,036 and U.S.Pat. No. 6,137,001.

The contacting step[s] can take place under oxidizing conditions, ifdesired. Suitable oxidizing conditions generally include a temperatureranging from about 125° C. to about 275° C. and a pressure ranging fromabout atmospheric, i.e. 0 MPa(g), to about 6 MPa(g) and a residence timeranging from about 5 seconds to about 2 weeks. That is, the mixture hasa temperature and a pressure within these ranges and may be maintainedwithin these ranges for a period of time within the residence timerange. In another embodiment, the temperature ranges from about 175° C.to about 225° C.; and the temperature may range from about 190° C. toabout 235° C. In an embodiment, the pressure ranges from about 1.2MPa(g) to about 6.0 MPa(g); and the pressure may range from about 1.5MPa(g) to about 6.0 MPa(g). In a further embodiment, the residence timeranges from about 10 minutes to about 12 hours. The oxidationtemperature, pressure and residence time may vary based on a variety offactors including for example, the reactor configuration, size, andwhether the process is, batch, continuous, or semi-continuous. Anoxidation condition may also vary based on other oxidation conditions.For example, use of a particular temperature range may enable use of adifferent residence time range.

In an embodiment, the terephthalic acid produced by the instantinvention may precipitate, crystallize, or solidify in a liquid phasemixture at the oxidation conditions and/or as the mixture cools. Thus, amixture according to the invention may further comprise solidterephthalic acid. Other compounds, including color bodies, and otheroxidation products may solidify with or be trapped in the solidoxidation product thus reducing the purity of the desired product. In anembodiment, the mixture comprises a liquid phase. The mixture maycomprise a gas phase such as when the oxidizing agent is added as a gas.The mixture may comprise a solid phase e.g. a mixture component, anoxidation product, or a by-product fails to dissolve or solidifies inthe mixture. In an embodiment, the mixture comprises a liquid phase, asolid phase and optionally a gas phase. In another embodiment, themixture comprises a liquid phase and a gas phase.

As noted above and discussed below, it has been discovered that theinvention may be used to produce an oxidation product having differentamounts of contaminants relative to those observed in conventionalprocesses. In addition, the invention provides new ways to control thelevel of various contaminants in the oxidation product. In anembodiment, a process according to the invention further comprisesforming the oxidation product as a solid, optionally at the oxidizingconditions, to produce the solid oxidation product and a mother liquor.The solid oxidation product may be separated from the mother liquor,i.e. liquid phase, and the mother liquor of the process may be recycledand reused in the contacting step or other steps of the processdescribed below.

Processes according to the invention, may comprise one or moreadditional oxidizing steps. In an embodiment, a second oxidation stepincludes a second oxidizing temperature that is lower than thetemperature of the first oxidizing step. Processes according to theinvention may include additional contacting steps of the invention asdescribed herein, and/or the invention may be combined with otheroxidizing steps such as conventional oxidizing steps known in the art.Multiple contacting and/or oxidation steps may be conducted in seriesand/or parallel and may be combined with other process steps such aspurification steps described herein.

In another embodiment, the invention further comprises purifying theoxidation product. Purifying may comprise one or more additional stepsto isolate and purify the oxidation product. Examples of purifying stepsinclude: separating wherein the oxidation product is separated from themother liquor or another liquid phase such as by filtration and/orcentrifugation; washing wherein the oxidation product is washed, forexample with water and/or another solvent component; drying theoxidation product; and hydrogenation processes. Although hydrogenationprocesses can be used for purification, they are less desirable thanother purification methods due to the cost. Such additional processingsteps have been described in the general literature and are well knownto those of ordinary skill in the art to be used in various combinationsto purify oxidation products of the invention. See for example, thereferences cited in this application and the art cited therein.

A purification step of the instant invention may further comprise one ormore solvent contacting steps. A solvent contacting step comprisescontacting an oxidation product, also including washed or dried solidoxidation products, with a third solvent comprising at least one ofwater, a carboxylic acid, an ionic liquid and/or ionic liquid precursor,and a mother liquor to produce a purified oxidation product. In anembodiment, the solvent of the solvent contacting step contains ionicliquid and carboxylic acid, and optionally mother liquor. Thecomposition of the solvent for the solvent contacting step can be asdescribed above for the contacting step.

Solvent contacting may leach impurities from the solid oxidationproduct, and/or the oxidation product may be partially or completelydissolved in the solvent. Solvent contacting conditions include asolvent contacting temperature. The solvent contacting temperature maybe lower than the oxidation temperature. In an embodiment, the solventcontacting temperature is at least 20° C. lower than the oxidationtemperature. Solvent contacting may be practiced for example in the oneor more crystallizers that follow the oxidation reactor in someconventional processes. The oxidation product may solidify, precipitate,or crystallize in the solvent of the solvent contacting step.

The product made by the process, either initially or following one ormore additional oxidizing and/or purification steps, can contain lessthan about 2500 ppm 4-CBA, or less than about 2000 ppm 4-CBA, or lessthan about 1500 ppm 4-CBA, or less than about 1000 ppm 4-CBA, or lessthan about 750 ppm 4-CBA, or less than about 500 ppm 4-CBA, or less thanabout 250 ppm 4-CBA, or less than about 100 ppm 4-CBA, or less thanabout 50 ppm 4-CBA, or less than about 25 ppm 4-CBA.

It should be noted that the terms “first”, “second”, and “third” etc.are being used to distinguish one component, or composition, or stage,or zone, or reactor etc. from another. It is not necessarily the casethat a “second” stage or zone, for example, physically or temporallyfollows a “first” stage or zone. Depending on the context, it could bebefore or after, as would be understood by those of skill in the art.

EXAMPLES

The examples are presented to further illustrate some aspects andbenefits of the invention and are not to be considered as limiting thescope of the invention.

Example 1 Experimental Procedure

In a fume hood, load a Parr reactor with the specified amounts ofcomponents for the given experiment seal the reactor. The Parr reactorincludes a gas distributor to disperse the gas through a 1.6 mm openinginto the liquid, a mechanical gas entrainment stirrer, and baffles toensure thorough mixing. Install the Parr reactor in a heater assembly atroom temperature and connect a gas supply line to the reactor and acondenser to the reactor outlet. During operation, gases exit thereactor through the condenser then a trap, then a back-pressureregulator. Connect a safety vent having a rupture disk, andthermocouples to the reactor. Connect a cooling water recirculator tothe condenser and begin to recirculate cooling water. Pressure test theParr reactor at room temperature and 1.4 MPa(g) (200 psig) usingnitrogen until there is no decrease in pressure for 15 minutes. Set theback pressure regulator on the reactor outlet to the experimentalpressure and pressure test the reactor under nitrogen.

Begin raising the reactor temperature to the experimental temperatureunder the nitrogen atmosphere. Always follow all instructions for thespecific reactor including temperature and pressure limits. When thereactor reaches the desired temperature begin adding air at theexperimental rate and monitor the reactor temperature and pressure forthe duration of the test. During the test, the air flow into the reactoris maintained at 2500 standard cm³ per minute, the pressure ismaintained at 4.1 MPa(g), and the stirrer is maintained at 1600 rpm. Atthe end of the test shut off the heater, cut the air flow and allow thereactor to cool. When the reactor cools to less than about 35° C., openthe back pressure valve, stop the cooling water, and remove and emptythe reactor to obtain the solid oxidation product and mother liquor.

The mother liquor and products are filtered under vacuum to separate thesolids and liquid. The solids are then mixed with approximately 100 ccdeionized water at room temperature and decanted. The room temperaturedeionized water mixing and decanting is repeated two additional times. Afourth wash with deionized water is heated to approximately 95° C. for30 minutes and then filtered. The solids are dried at 80° C. for 8-24hours before analyzing.

Examples 2-3

Examples 2-3 were individual tests conducted using the equipment andprocedure given in Example 1. The components of the mixture, given ingrams, operating temperature, time, and air flow, and results are givenin Table 1.

Example 2

p-Xylene used as the starting material under oxidizing conditions.

Example 3

Same oxidizing conditions as Example 2 except p-toluic acid was used asthe starting material. The amount of 4-CBA, and p-toluic acid in theproduct dropped significantly, and the benzoic acid dropped slightly, asshown in FIG. 2. The recovery and the selectivity increased.

Having described the invention in detail and by reference to specificembodiments thereof, it will be apparent that modifications andvariations are possible without departing from the scope of theinvention defined in the appended claims. More specifically, althoughsome aspects of the present invention are identified herein as preferredor particularly advantageous, it is contemplated that the presentinvention is not necessarily limited to these preferred aspects of theinvention.

TABLE 1 Example number 2 3 Oxidation Temperature, ° C. 215 215 OxidationTime 3 3 Air Flow, sccm 2500 2500 Cooling Fast Fast Components (g)p-Xylene 20 p-Toluic Acid 20 Acetic Acid 44 44 BMImOAc 20 20 BMImBr 1616 NH₄OAc 12 12 HBr 0.4 0.4 H₂O 0.4 0.4 Co(OAc)₂—4H₂O 0.8 0.8 Mn(OAc)₂0.6 0.6 Analysis of Product Terephthalic acid (%) 97.2 99.1 4-CBA (ppm)1081 230 Benzoic acid (ppm) 1163 1143 p-Toluic acid (ppm) 25,607 7183Visual Product Color White White Recovery (%) 85.2 91.9

What is claimed is:
 1. A process for producing terephthalic acidcomprising: providing a p-xylene stream enriched with p-toluic acid;contacting the p-xylene stream enriched with p-toluic acid, a solventcomprising an ionic liquid, a bromine source, a catalyst, and anoxidizing agent to produce a product comprising terephthalic acid. 2.The process of claim 1, wherein in the providing step, the p-xylenestream comprises greater than about 5% by weight p-toluic acid.
 3. Theprocess of claim 1, wherein the solvent further comprises a carboxylicacid.
 4. The process of claim 1, wherein the solvent has a ratio of thecarboxylic acid to the ionic liquid within a range of about 1:16 to 16:1by weight.
 5. The process of claim 1, wherein the contacting step isperformed under oxidation conditions.
 6. The process of claim 1, whereinthe catalyst comprises at least one of cobalt, titanium, manganese,chromium, copper, nickel, vanadium, iron, molybdenum, tin, cerium, andzirconium.
 7. The process of claim 1 wherein the ionic liquid is formedin situ from at least one ionic liquid precursor wherein the contactingstep is performed in a reaction zone.
 8. The process of claim 1 whereina cation of the ionic liquid is formed from imidazoles, pyrazoles,thiazoles, isothiazoles, azathiozoles, oxothiazoles, oxazines,oxazolines, oxazaboroles, dithiozoles, triazoles, selenozoles,oxahospholes, pyrroles, boroles, furans, thiphenes, phospholes,pentazoles, indoles, indolines, oxazoles, isothirazoles, tetrazoles,benzofuran, dibenzofurans, benzothiophenes, dibenzothoiphenes,thiadiazoles, pyrdines, pyrimidines, pyrazines, pyridazines,piperazines, piperidines, morpholines, pyrans, annolines, phthalazines,quinazolines, quinoxalines, or combinations thereof.
 9. The process ofclaim 1 wherein an anion of the ionic liquid is halides, borides,phosphates, arsenates, stibates, acetate, carboxylates, azolates,sulfates, acyl units, CO₃ ²⁻; NO₂ ¹⁻, NO₃ ¹⁻, SO₄ ²⁻, PO₄ ³⁻, (CF₃)SO₃¹⁻, derivatives thereof, or combinations thereof.
 10. A process forproducing terephthalic acid comprising: providing a p-xylene streamcomprising p-toluic acid, wherein the p-xylene stream is produced by aprocess in which the p-toluic acid is maintained at a level of at leastabout 5% by weight of the p-xylene stream produced therefrom; contactinga solvent comprising an ionic liquid, a bromine source, a catalyst, thep-xylene stream of the providing step, and an oxidizing agent, toproduce a product comprising terephthalic acid.
 11. The process of claim10, wherein the solvent further comprises a carboxylic acid.
 12. Theprocess of claim 10, wherein the solvent has a ratio of the carboxylicacid to the ionic liquid within a range of about 1:16 to 16:1 by weight.13. The process of claim 10, wherein said contacting step is performedunder oxidation conditions.
 14. A process for producing terephthalicacid comprising: contacting p-xylene, a first solvent, a first brominesource, a first catalyst, and a first oxidizing agent to produce ap-xylene stream enriched with p-toluic acid; maintaining the p-toluicacid at a level of at least about 5% by weight of the p-xylene streamenriched with p-toluic acid; contacting the p-xylene stream enrichedwith p-toluic acid, a second solvent comprising an ionic liquid, asecond bromine source, a second catalyst, and a second oxidizing agentto produce a product comprising terephthalic acid.
 15. The process ofclaim 14, wherein the solvent comprises a carboxylic acid.
 16. Theprocess of claim 14, wherein the contacting steps are done underoxidation conditions.
 17. The process of claim 14, wherein the catalystcomprises at least one of cobalt, titanium, manganese, chromium, copper,nickel, vanadium, iron, molybdenum, tin, cerium, and zirconium.
 18. Theprocess of claim 14, wherein the contacting steps are performed in areaction zone.
 19. The process of claim 14 wherein contacting p-xylene,a first solvent, a first bromine source, a first catalyst, and a firstoxidizing agent is performed in a first reaction zone, and whereincontacting the p-xylene stream enriched with p-toluic acid, a secondsolvent comprising an ionic liquid, a second bromine source, a secondcatalyst, and a second oxidizing agent is performed in a second reactionzone.
 20. The process of claim 19 wherein the first reaction zone andthe second reaction zone are in the same reactor or in differentreactors.